<!DOCTYPE html>
<html>
<body>
  <input type="text" id="num1" value="12">
  <select id="op">
    <option value="*">*</option>
  </select>
  <input type="text" id="num2" value="24">
  <button onclick="calculate()">=</button>
  <input type="text" id="result" value="288">
  <script>
    function calculate() {
      const num1 = parseFloat(document.getElementById('num1').value);
      const op = document.getElementById('op').value;
      const num2 = parseFloat(document.getElementById('num2').value);
      let result;
      if (op === '*') {
        result = num1 * num2;
      }
      document.getElementById('result').value = result;
    }
  </script>
</body>
</html>